I bought the PRO license, because I liked the feature to use SQL data compare on a SQL backup file directly. And that is only in the PRO license. However, now I can't get it to work with any backup file. I tried many different backup files and everytime I hit "compare now", it starts, but then after a few seconds, it stops with the message "unable to open backup file". No other message whatsoever, just that.

The strange thing is, when I open the BAK file, it immediately tells me the info in the start-screen. It shows me that the file contains a full backup, data and the correct database. But then, when I start comparing, it fails.

Can anybody tell me what I have to check to make SQL backup files work with SQL data compare PRO? Maybe specific security settings?

Does the error always happen at the same point with whatever backup file is used? If the backup is situated on a network share, my initial guess would be that there was some kind of network glitch that caused the connection to drop. there isn't anything in the tool to recover from a network outage, so this might be a possibility.

Would you be able to try using SQL Virtual Restore to mount the backup file?
<http://www.red-gate.com/products/dba/sql-virtual-restore/>

This will probably give you better results, as it will allow you to compare the backup file as a live database, rather than the slightly more limited comparison you can do directly through SQL Data Compare backup reader.